One is to say:

The passage from potentiality to actuality either requires an external cause or does not. If it does not, this argument is invalid. If it does, then it requires either an external efficient cause, a final cause, or both.

Dependence upon a final cause alone is insufficient. For if what is potential passes into actuality only through an external cause, there must be a cause for its being an agent. Otherwise, the object of love alone, without that by which the lover brings about what he seeks, does not necessitate the occurrence of the act.

Thus, if the lover is incapable of acting, he does not move towards the object of his love. His movement towards it is contingent: neither impossible nor necessary in itself. Neither alternative concerning a contingent thing can prevail without a complete determining cause that necessitates its existence. The agency of a contingent thing therefore requires a determining cause complete in its efficacy, and that is the act, since the end alone is not a complete determining cause.

If the passage from potentiality to actuality requires an external cause—an efficient cause, or both an efficient and a final cause—he may be told:

The motion of corporeal substance then requires an agent external to that motion. In that case, this agent cannot have been responsible for that agency from eternity. Otherwise, all motions and newly occurring events would necessarily have existed from eternity, given the existence of their agent with its agency complete. It must therefore have become the agent of those newly occurring events after not having been their agent.

Thus, if the contingent thing is held to require an external cause, it follows that this cause became a cause of its agency after not having been such a cause.

This may also be used as an argument concerning causality in general, whether efficient or final. For it may be said:

For every newly occurring event, it is impossible for its complete cause to have existed from